This proposal outlines a two-year transitional grant for Jesse D. Bloom, Ph.D. The grant would be activated once Dr. Bloom begins an assistant professor position at a research institution, and would be used to fund the initial years of his work. Dr. Bloom is currently a postdoctoral scholar in the laboratory of Dr. David Baltimore at the California Institute of Technology. Dr. Bloom's research examines molecular constraints on viral evolution. This specific proposal deals with the evolution of oseltamivir (Tamiflu) resistance in H1N1 influenza viruses via the His274->Tyr (H274Y) mutation to the neuraminidase protein. This resistance mutation had long been thought unlikely to spread since it attenuated seasonal H1N1 viruses, but it recently spread worldwide. Dr. Bloom's preliminary research has shown that this global spread of H274Y was enabled by secondary "permissive" mutations that rescued a defect that H274Y caused in the surface expression of neuraminidase protein. The proposed research will examine in detail the biophysical defect caused by H274Y, and the mechanism by which it affects viral growth in tissue culture and animal models. In order to aid in the identification of other possible permissive mutations that might enable the evolutionary spread of H274Y, the research will develop and test computational prediction methods. Finally, these computational and experimental tools will be used towards forecasting the potential for H274Y to confer widespread oseltamivir resistance on the swine-origin 2009 pandemic H1N1 influenza strain. The proposed work will therefore represent a substantial step towards the goal of predicting virus evolution. Progress towards this goal will be of public health value in regards to medically important viruses such as influenza. Public Health Relevance: Viral diseases such as influenza are so problematic because they rapidly evolve to escape immunity and antiviral drugs. This proposal examines a specific example of viral escape, the evolution of oseltamivir (Tamiflu) resistance in H1N1 influenza. The goal is to understand this escape at a molecular level, and use the resulting insight to anticipate potential future routes of oseltamivir resistance.